The invention relates to a detector for the interaction products, particularly backscattered electrons and secondary electrons, produced in a particle beam device by interaction of a primary beam with a sample to be investigated.
Usually used for the detection of secondary electrons or backscattered electrons in scanning electron microscopes are so-called Everard-Thornley detectors (ETD), in which the secondary electrons or backscattered electrons released at the sample interface are attracted away from the sample to the detector by an extraction grid and then accelerated to a scintillator having a high voltage of about 10 kV. When the highly kinetic electrons strike the scintillator, photons are produced which can be fed to a photodetector, for example a photomultiplier, by means of a transparent light guide.
Such Everard-Thornley detectors cannot be used with varying gas pressures in the region of the detector, particularly when the ambient pressure of the detector is above 10−3 hPa, since the increased conductivity of the residual gas leads to overstrikes at the high voltage applied to the scintillator.
At pressures above 10−3 hPa in the sample chamber, for indirect detection of the secondary electrons released by the primary beam, an extraction potential of up to 400 V is applied to an electrode in order to accelerate the released secondary electrons away from the sample. A gas cascade thereby results from collisions of the secondary electrons. Further, tertiary electrons arise in this gas cascade, and also photons from scintillation effects. Signal detection then takes place either by the measurement of the electron current or by the detection of the photons. Corresponding detection principles are described, for example, in U.S. Pat. No. 4,785,182 and WO 98/22971.
Devices which are designed for operation at varying pressure conditions in the sample chamber, and in which the electron microscopic investigation of samples can thus take place both under ultra-high vacuum conditions and also under so-called ambient conditions, in which the pressure in the sample chamber is above 10−3 hPa, have to have different detectors for the different modes of operation.
From Japanese Patent Document P 11-096956 A, a detector for scanning electron microscopes is known in which the same photodetector is used both for detection of cathode luminescence and also for the detection of backscattered electrons. For this purpose, the detector has a scintillator attached to a light guide and having a convex, mirror-coated end. The backscattered electrons penetrate into the mirror layer and produce light flashes in the scintillator which are detected by the photodetector; cathode luminescence, on the other hand, is focused by the reflecting surface of the scintillator onto another light inlet surface of the light guide.
In this detector it is however disadvantageous that the scintillator has to be arranged between the sample and the objective lens of the scanning electron microscope, so that a correspondingly greater working distance is necessary between the objective lens and the sample; because of the resulting scattering of the electrons by gas molecules, this detector is unsuitable for uses with high pressures in the sample chamber. Moreover, the scintillator is disturbed by a tilting of the sample.
An Everard-Thornley detector is described in German Patent Document DE 40 09 692 A1, its surface being provided with a metal grid. The metal grid acts, among other things, to prevent the presence of surface charges on the non-conducting scintillator. A use of the detector at different pressures in the sample chamber is not mentioned there.